An idf is continuous per corpus, and accounts for your ratio of documents that include the word "this". With this case, We have now a corpus of two documents and all of them consist of the term "this".log ⁡ N n t = − log ⁡ n t N displaystyle log frac N n_ t =-log frac n_ t N Tf–idf is closely linked to the damaging logarithmically transform